hgweb: fix type of {nav} keyword when linerange filter is active
Spotted by making '%' operator check the operand type more strictly. Before,
"{nav}" would be evaluated to a lazy "" (empty string) and {"" % x} would
yield one empty string.
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..